Barcelona

I'm going to Spain in 13th and will be in Barcelona the following week. I have the same question.

First, a heads up: you posted under Hotel and Restaurant Reviews; this is where you would tell other travelers about places you stayed or dined. You should post questions under Destination Q&A. That's where travelers look to help out other travelers.

I had two experiences with demonstrations in Europe and both were in Paris. The first occurred when I was on my own. The demonstration was peaceful but blocked my route to the Jardin du Luxembourg, so to be safe, I just walked away when I realized what was going on and took a different route. There were numerous police nearby around the corner waiting in the event they were needed; they weren't.
The second time I was on a Rick Steves tour. The demonstration was not so peaceful, but our alert guide, Elisabeth van Hest, simply adjusted her plans for the evening and we went elsewhere to enjoy Paris.

I doubt either the police or the separatists will bother tourists, but some areas could be blocked off.

I live in Barcelona and actually people from here are not really aggressive. There were manifestations and they keep doing some now from time to time but in fact it was not really dangerous and people were not attacking anyone or something.